The present invention relates to hydraulic hybrid vehicle systems. Hydraulic hybrid vehicle systems include four main components: a hydraulic fluid, a fluid reservoir, a hydraulic pump/motor (in parallel hybrid systems) or in-wheel motors and pumps (in series hybrid systems), and a fluid accumulator. The pump/motor extracts kinetic energy during braking to pump the working fluid from the fluid reservoir to the fluid accumulator. Working fluid is thus pressurized. When the vehicle accelerates, this pressurized working fluid provides energy to the pump/motor to power the vehicle.
As is known, hydraulic pumps generate a substantial amount of noise. Unavoidable design imperfections of the components used in hydraulic pumps result in uneven flow characteristics and pressures waves that produce unwanted system noise. In additional, these uneven flow characteristics and pressure waves make the components vibrate. This vibration of system components also generates unwanted system noise. Higher pump flow rates lead to an increase in uneven flow characteristics, pressure waves, and system component vibration. Accordingly, higher pump flow rates generate a significant amount of unwanted hydraulic system noise.